Cereblon (CRBN), a component of the DDB1-CUL4a-Roc1 ubiquitin ligase complex, has been identified as a target of certain immunomodulatory compounds, e.g., thalidomide, lenalidomide, and pomalidomide (Lopez-Girona et al., Leukemia 2012). It is believed that the interactions of CRBN with the immunomodulatory compounds mediate their anti-proliferative effects in multiple myeloma (MM) cells (Lopez-Girona et al., Leukemia 2012; Zhu et al., Blood 2011, 118, Abstract 127).
The translated full-length CRBN protein contains 442 amino acid residues. It has been established that the C terminal region of CRBN binds the immunomodulatory compound, thalidomide, and that amino acid residues 374Y and 376W are critical for the binding of thalidomide to CRBN (Ito et al., Science 2010, 327, 1345-1350).
CRBN is encoded by a 25 kb gene on chromosome 6, consisting of 11 exons and 10 introns. Thus, alternative splicing process can potentially generate multiple functional proteins as well as variants of a protein from a single gene having different structural organization and functional activity. Truncated proteins that have lost interaction domains or critical functional amino acid residues may create non-functional or aberrant CRBN proteins that may interfere with the functions of the full-length CRBN protein and reduce or alter the therapeutic activity of a treatment compound that exerts its activity via its interactions with the full-length CRBN protein.
At least two isoforms of the protein cereblon (CRBN) exist, which are 442 and 441 amino acids long, respectively, and CRBN is conserved from plant to human. In humans, the CRBN gene has been identified as a candidate gene of an autosomal recessive nonsyndromic mental retardation (ARNSMR). See Higgins, J. J. et al., Neurology, 2004, 63:1927-1931. CRBN was initially characterized as an RGS-containing novel protein that interacted with a calcium-activated potassium channel protein (SLO1) in the rat brain, and was later shown to interact with a voltage-gated chloride channel (CIC-2) in the retina with AMPK1 and DDB1. See Jo, S. et al., J. Neurochem, 2005, 94:1212-1224; Hohberger B. et al., FEBS Lett, 2009, 583:633-637; Angers S. et al., Nature, 2006, 443:590-593. DDB1 was originally identified as a nucleotide excision repair protein that associates with damaged DNA binding protein 2 (DDB2). Its defective activity causes the repair defect in the patients with xeroderma pigmentosum complementation group E (XPE). DDB1 also appears to function as a component of numerous distinct DCX (DDB1-CUL4-X-box) E3 ubiquitin-protein ligase complexes which mediate the ubiquitination and subsequent proteasomal degradation of target proteins. CRBN has also been identified as a target for the development of therapeutic agents for diseases of the cerebral cortex. See WO 2010/137547 A1.
As a direct binding target of immunomodulatory agents, CRBN can thus potentially be used as a biomarker for a treatment with an immunomodulatory agent, such as thalidomide, lenalidomide, pomalidomide, 3-(5-amino-2-methyl-4-oxoquinazolin-3(4H)-yl)piperidine-2,6-dione, or (S)-3-(4-((4-(morpholinomethyl)-benzyl)oxy)-1-oxoisoindolin-2-yl)piperidine-2,6-dione.